How exactly should I set surface normals for a sculpture?
In Edit Mode, you can display normals, to see exactly which dir they are facing (N > Mesh Display).



You might be able to simply call normals_make_consistent() operator:

bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all()
bpy.ops.mesh.normals_make_consistent()


Some models don't always import with consistent normals, because they weren't consistent with the order of their vertex arrangement (clockwise sometimes, counter-clockwise elsewhere, within the same model)